The invention relates to a lead silicate glass and to the use thereof for the production of secondary electron multipliers. The invention furthermore relates to a process for setting a reduced surface resistance of such lead silicate glasses according to the invention.
Secondary electron multipliers (SEMs) are, besides the photocathode, the main component of photomultipliers. The principle of operation of this electronic component is based on the outer photo effect, in which mobile charge carriers are generated through interaction of material (photocathode) with radiation (IR, visible and UV light, X-rays or radioactive radiation). Conventional photomultipliers consist of an evacuated glass tube (bulb) containing the photocathode and an anode. A number of electrodes, known as dynodes, are arranged between the cathode and the anode. The photoelectrons generated by irradiation with photons (or other particles of suitable energy) are accelerated by means of a voltage gradient between the electrodes and hit the dynode surface with a kinetic energy of about 100 eV and release a number of secondary electrons. Each electron produces a cascade of secondary electrons in this way. The photocurrent can thus be amplified by up to 109 times. The function of the discrete dynodes can be taken over by a continuous glass tube surface if the latter has a suitable surface resistance, for example through coating or reduction. A photomultiplier of this type is described in the U.S. Pat. No. 6,166,365.
Open SEVs (without a photocathode) are furthermore employed for detecting electrons and ions, for example in mass spectrometers and residual gas analysers.
With a low surface resistance, better current flow can be ensured, which in turn guarantees optimum subsequent delivery of the released secondary electrons and enables an increase in the dynamic range. However, if the surface resistance is too low and thus the electrical conductivity is too high, undesired warming of the material occurs or even destruction thereof, for example through thermal stresses.
Besides a high secondary electron yield and a high amplification factor, a low background noise of the photomultiplier is necessary in order, for example on use in a gamma-ray camera for medical purposes, to achieve good image resolution. The background noise is caused by naturally occurring radioactive isotopes, such as potassium 1940K and rubidium 3787Rb, which are used, for example, as a constituent of glasses and contribute towards undesired formation of secondary electrons.
Lead silicate glasses and the use thereof for secondary electron generation have been known for some time.
Thus, GB 1,239,687 describes a lead- and bismuth-containing silicate glass of the composition (in % by weight) SiO2 30-70, PbO 6-30, Bi2O3 2-45, Al2O3 0.5-10, MgO 0.5-7, and the optional components B2O3=5, Na2O=6, K2O=10, CaO+SrO=8, As2O3+Sb2O3=2. Likewise described is a dynode containing a plurality of glass channels produced from the said glass. A striking feature of this glass composition, besides the relatively low PbO proportion, is the wide Bi2O3 range with a relatively high upper limit.
PbO and Bi2O3 affect the surface resistance of the glass after reduction in the same way. However, one component cannot be replaced by the other. The combination of PbO and Bi2O3 within the abovementioned ranges results only in a limited range of the achievable surface resistance. Through the content of K2O, the glass is of only limited suitability for the production of very sensitive secondary electron multipliers. The glass examples given all contain K2O in the range from 0.6 to 7.5% by weight. Due to the background noise associated therewith, the sensitivity of the dynodes containing the glass is significantly reduced.
The publication DE 33 177 78 A1 describes a glass which is suitable for the production of micro-channel plates which are used as secondary electron multipliers. A composition range (in mol %) of SiO2 63-72, PbO 20-30, alkali metal oxides 3-7, alkaline earth metal oxides 1-3.5 and Al2O3, Bi2O3, Al2O3 less than 1 is given for the glass. In this glass too, its surface resistance can only be set in a limited range. Due to the content of alkali metal oxides, the glass is of only limited suitability for the production of very sensitive secondary electron multipliers.
A similar situation applies to the glass claimed in the specification GB 2,218,982 A of the composition (in % by weight) SiO2 30-35, PbO 50-57, Cs2O 2-10, MgO+CaO+SrO+BaO 0-5, Al2O3+ZrO2+TiO2+Nb2O5 0.1-1, and a molar ratio between SiO2 and PbO of 2.0-2.4. Particularly low surface resistance values cannot be achieved through the said composition. High Cs2O contents likewise increase the achievable surface resistance greatly, meaning that, in overall terms, the resultant current flow cannot ensure the electrode push in the SEM.
A further lead silicate glass which is suitable for the generation of secondary electrons is described by the specification SU 17 175 66 A1. Besides SiO2 and PbO, the glass contains 1-15 mol % of BeO. The use of BeO is extremely questionable for toxicological reasons. Furthermore, the surface resistance of this glass can again only be set in a limited range.
Furthermore, the specification JP 88 166 735 discloses a lead silicate glass having the composition (in % by weight) SiO2 15-65, PbO 15-75, Al2O3 0-10, CS2O 0.1-50, Li2O 0-5, Na2O 0-10, K2O 0-20, Rb2O 0-30, Cs2O+Li2O+Na2O+K2O+Rb2O 0.1-50, B2O3 0-20, Al2O3 0-10, MgO 0-10, CaO 0-10, SrO 0-20, BaO 0-25, ZnO 0-15, CdO 0-10, MgO+CaO+SrO+BaO+ZnO+CdO 0-25, Bi2O3 0-20, Sb2O3 0-25, Tl2O 0-30, Bi2O3+Sb2O3+Tl2O 0-30, TiO2 0-10, WO3 0-7, As2O3 0-2, T2 0-10, where at least one or two of the oxides Bi2O3, Sb2O3, Tl2O, WO3 or As2O3 must be present.
The specification JP 91 295 828 describes a glass containing heavy-metal oxides and rare-earth oxides. The heavy-metal oxides are PbO, Bi2O3, CdO, Ga2O3, TeO2, Sb2O3, As2O3 and GeO2. In the case of the two last-mentioned glasses, it is virtually impossible to foresee what effect the multiplicity of components mentioned will have on the surface resistance. In any case, targeted setting of the surface resistance is difficult. In addition, the use of a large number of components, which are frequently expensive, increases the production costs.
The object of the invention is to find a lead silicate glass. A glass should be simple to process and nevertheless have good heat stability. It should be possible to set a low surface resistance of the glass, and the glass should be suitable for the production of secondary electron multipliers having a stable secondary electron yield, a high amplification factor and low background noise. In addition, the resultant glass surface should be as smooth as possible in order substantially to avoid unevenness of the surface, which results in field emission and impairs the sensitivity of the secondary electron multipliers.
This object is achieved by a lead silicate glass, characterized by a composition by % weight, 15-35% SiO2, 35-55% PbO,  greater than 20-29 Bi2O3, 0-10 BaO, 1-10 Cs2O, 2-13 BaO+Cs2O, 0-10 CaO, 0-10 SrO, 0-10 CaO+SrO+BaO, and up to 1% by weight of fining agents, the use of said glass for production of secondary electron multipliers, and by a process for setting a reduced surface resistance of said glass, characterized in that the lead silicate glass is exposed to a reducing hydrogen atmosphere, during which a certain surface resistance is set under defined reduction conditions, depending on the composition of the lead silicate glass.
In contrast to conventional lead silicate glasses, the glass according to the invention is distinguished by a proportion of  greater than 20-29% by weight of Bi2O3. Owing to its good processing properties, its nevertheless high heat stability and its particular properties, the glass is highly suitable for the production of secondary electron multipliers. Thus, the surface resistance of the glass can be set in a targeted manner in a particularly low and thus favourable region by reduction in the hydrogen atmosphere. The SEM component produced from the glass according to the invention has, after reduction of this glass, a stable secondary electron yield and a high amplification factor.
The lead silicate glass according to the invention contains less than 2% by weight of K2O, in particular less than 1% by weight of K2O and preferably less than 0.05% by weight of K2O. The lead silicate glass is particularly preferably potassium-free, apart from unavoidable amounts as present, for example, in the glass raw materials or in the wall material of a melting unit.
Due to the fact that glass components containing radioactive isotopes, such as K2O and Rb2O, have been omitted, the SEM component has extremely low background noise. The glass withstands demanded temperature loadings in an excellent manner.
The glass according to the invention contains 15-35% by weight of SiO2 and 35-55% by weight of PbO as network-forming constituents. At lower contents than 15% by weight of SiO2, the glass transition temperature Tg drops. The temperature range between Tg and the softening point likewise reduces. Such short glasses have poor processing properties. In addition, only low operating temperatures of the SEM are then permissible in order to prevent thermal deformation of the component. SiO2 contents of greater than 35% by weight result in disadvantageously high processing temperatures, which impair, in particular, the subsequent processing of the glass for the production of SEMs. The glass transition temperature (Tg) of the glass is preferably greater than 430xc2x0 C. in order to make the SEM resistant to thermal deformations. The softening temperature, at which the glass can deform due to its inherent weight, is more than 100xc2x0 C. above the glass transition temperature. Optimum process control in the production of the SEMs is thus ensured. An excessively high SiO2 content ( greater than 35% by weight) furthermore adversely affects the crystallization tendency, making the surface nature of the glass unsuitable for the desired application.
The PbO constituent of 35-55% by weight, together with Bi2O3, which is present in an amount of  greater than 20-29% by weight, determines the surface resistance after reduction of the glass surface, for example in a hydrogen atmosphere. The two components (Bi2O3 and PbO) are vital in the reduction process for formation of a semiconducting layer on the surface of the glass, which is distinguished by a low surface resistance. Less than 20% by weight of Bi2O3 causes an increase in the surface resistance after reduction so that this is outside the desired range. At a Bi2O3 content above 25% by weight, the surface resistance after reduction drops so much that, on use as SEM, very high currents and thus undesired warming can occur, impairing the functioning of the SEM or even destroying the component. The same applies to PbO. If the PbO proportion drops below 35%, an undesired increase in the surface resistance occurs. If the PbO proportion exceeds 55% by weight, by contrast, a considerable reduction in the surface resistance is caused.
The glass contains BaO and Cs2O as network modifiers. The same job is done by CaO and SrO, which can additionally be employed if desired. The total alkaline earth metal content of the glasses should not exceed 10% by weight in order to ensure an appropriate combination of the physical properties, such as low thermal expansion xcex120/300 and the desired surface resistance. The use of Cs2O is particularly advisable if it is desired to avoid an excessive lowering of the surface resistance after reduction. The glass should contain in total from 2 to 13% by weight of BaO and Cs2O.
The glass may in addition contain conventional fining agents, such as, for example, As2O3 and Sb2O3, which are used in conventional proportions of up to 1% by weight.
After reduction in a hydrogen atmosphere, the surface resistance of this glass is in the range from 0.1 to 60 Mxcexa9/sq.
A lead silicate glass according to the invention having the composition (in % by weight) SiO2 20-30, PbO 38-52, Bi2O3 21-26, BaO 0-9, Cs2O 0.1-9, BaO+Cs2O 3-12, CaO 0-9, SrO 0-9, CaO+SrO+BaO 0-9, and with up to 1% by weight of conventional fining agents has, besides the above advantages, a surface resistance of reduction in a hydrogen atmosphere of from 0.2 to 40 Mxcexa9/sq.
By changing the glass composition and in particular the PbO and Bi2O3 contents, the desired surface resistance can be set in a targeted manner.
Thus, a glass, to which up to 1% by weight of conventional fining agents can be added, having a composition (in % by weight) of SiO2 21-30, PbO 39-51, Bi2O3 22-25, BaO 0-9, Cs2O 0.2-6, CaO 0-8, SrO 0-8, CaO+SrO+BaO 0-9, has a surface resistance in the range from 0.3 to 25 Mxcexa9/sq. A glass having the composition SiO2 22-29, PbO 40-50, Bi2O323-25, BaO 0-8, Cs2O 0.2-5, BaO+Cs2O 4-10, CaO 0-7, SrO 0-7, CaO+SrO+BaO 0-9, has, after reduction, a surface resistance of from 0.4 to 20 Mxcexa9/sq, and a glass having the composition SiO2 22-29, PbO 40-50, Bi2O3 23-24, BaO 0-8, Cs2O 0.2-5, BaO+Cs2O 5-10, CaO 0-6, SrO 0-6, CaO+SrO+BaO 0-8, has a surface resistance of from 0.5 to 6 Mxcexa9/sq.
In order to set a reduced surface resistance of a glass according to the invention, the glass is exposed to a reducing hydrogen atmosphere, during which a certain surface resistance is set, depending on the composition of the glass.
Besides the glass composition, the setting of the surface resistance by reduction of the glass on its surface plays an important role. Thus, the surface resistance can be set within the limits determined by the composition of the glass essentially by variation of temperature and reduction time within certain limits. The surface resistance passes through a minimum with increasing temperature. After a certain reduction time, a saturation value is reached. The optimum reduction conditions for setting a desired surface resistance for a glass of a certain composition can be determined experimentally in a simple manner.